Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

Alam, Tasmim; Wienold, Martin; Lü, Xiang; Biermann, K.; Schrottke, L.; Grahn, H. T.; Hübers, Heinz‐Wilhelm

doi:10.1364/oe.27.036846

“…These noise peaks are 40-60 dB below the main tone, which is lower than the phase noise of the multiplied microwave reference, making them a non-issue with regards to spectral resolution, but the amplitude fluctuations increase the averaging time necessary to measure small signals and would limit the sensitivity of a measurement assuming a given Allan variance time for the system. Amplitude stabilization of QCLs has been demonstrated using a number of external components, such as a tunable beam block controlled with a voice coil [46], injecting power from an infrared laser into the QCL substrate to tune the refractive index via excitation of electron-hole pairs [47], and using a graphene based split-ring resonator metasurface [48]. These approaches may be adaptable to the VECSEL, depending on the actuation speeds (particularly for a beam block) and potential coupling to frequency tuning, resulting in a more complex coupled system.…”

Section: Results: Phase-lockingmentioning

confidence: 99%

Phase locking of THz QC-VECSELs to a microwave reference

Curwen¹,

Kawamura²,

Hayton³

et al. 2023

Preprint

0

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<p>High resolution frequency study and phase-locking have been performed on terahertz quantum-cascade vertical-external-cavity surface-emitting-lasers at 2.5 THz and 3.4 THz. A subharmonic diode mixer is used to downconvert the THz signal to a gigahertz intermediate frequency (IF). Feedback from reflections at the mixer are observed to have a strong influence on the free-running QC-VECSEL frequency stability as a result of efficient coupling to free-space compared to more typical ridge waveguide lasers. Instabilities in feedback result in free-running linewidths of tens of MHz, and at times >100 MHz. The QC-VECSEL IF signal is phase locked to a 100 MHz reference using the bias on the device as a means of error correction. Between 90-95% of the QC-VECSEL signal is locked within 2 Hz of the multiplied RF reference, and amplitude fluctuations on the order of 1-10% are observed, depending on the bias point of the QC-VECSEL. The bandwidth of the locking loop is ~1 MHz. Many noise peaks in the IF signal corresponding to mechanical resonances in the 10 Hz-10 kHz range are observed. These peaks are generally -30 to -60 dB below the main tone, and are below the phase noise level of the multiplied RF reference which ultimately limits the phase noise of the locked QC-VECSEL.</p>

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“…A QCL operated in a He-flow cryostat, free from mechanical vibrations and with a temperature stability of 2.2 mK will correspond to a linewidth of 750 kHz [82]. If frequency stabilization is applied, the linewidth of the QCL can be reduced further to a few hundred kHz [83]. Several groups have demonstrated frequency locking of a THz-QCL using the centre frequency of a molecular absorption line such as methanol as a frequency [55,84,85].…”

Section: Terahertz Sourcesmentioning

confidence: 99%

A practical guide to terahertz imaging using thermal atomic vapour

Downes

¹

,

Torralbo-Campo

²

,

Weatherill

³

2023

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This tutorial aims to provide details on the underlying principles and methodologies of atom-based terahertz imaging techniques. Terahertz imaging is a growing field of research which can provide complementary information to techniques using other regions of the electromagnetic spectrum. Unlike infrared, visible and ultraviolet radiation, terahertz passes through many everyday materials, such as plastics, cloth and card. Compared with images formed using lower frequencies, terahertz images have superior spatial resolution due to the shorter wavelength, while compared to x-rays and gamma rays, terahertz radiation is non-ionising and safe to use. The tutorial begins with the basic principles of terahertz to optical conversion in alkali atoms before discussing how to construct a model to predict the fluorescent spectra of the atoms, on which the imaging method depends. We discuss the practical aspects of constructing an imaging system, including the subsystem specifications. We then review the typical characteristics of the imaging system including spatial resolution, sensitivity and bandwidth. We conclude with a brief discussion of some potential applications.

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“…A QCL operated in a He-flow cryostat, free from mechanical vibrations and with a temperature stability of 2.2 mK will correspond to a linewidth of 750 kHz [83]. If frequency stabilization is applied, the linewidth of the QCL can be reduced further to a few hundred kHz [84]. Several groups have demonstrated frequency locking of a THz-QCL using the center frequency of a molecular absorption line such as methanol as a frequency reference [85,56,86].…”

Section: Terahertz Sourcesmentioning

confidence: 99%

A practical guide to Terahertz imaging using thermal atomic vapour

Downes¹,

Torralbo-Campo²,

Weatherill³

2022

Preprint

0

View full text Add to dashboard Cite

This tutorial aims to provide details on the underlying principles and methodologies of atom-based terahertz imaging techniques. Terahertz imaging is a growing field of research which can provide complementary information to techniques using other regions of the electromagnetic spectrum. Unlike infrared, visible and ultraviolet radiation, terahertz passes through many everyday materials, such as plastics, cloth and card. Compared with images formed using lower frequencies, terahertz images have superior spatial resolution due to the shorter wavelength, while compared to x-rays and gamma rays, terahertz radiation is non-ionising and safe to use. The tutorial begins with the basic principles of terahertz to optical conversion in alkali atoms before discussing how to construct a model to predict the fluorescent spectra of the atoms, on which the imaging method depends. We discuss the practical aspects of constructing an imaging system, including the subsystem specifications. We then review the typical characteristics of the imaging system including spatial resolution, sensitivity and bandwidth. We conclude with a brief discussion of some potential applications. Introduction. Using Atoms as SensorsAtoms in dilute vapour can make very effective sensors [1,2]. Atoms have no moving parts to wear out, they are relatively unperturbed by inter-atomic interactions, their energy levels are sensitive to applied fields, and crucially, each atom of the same isotope is identical. This final point means that atomic sensors are in effect pre-calibrated and measurements made using them are reproducible and should be, at least in principle, traceable to the SI system of measurements. Atomic systems already provide a platform for precision clocks [3,4], gyroscopes [5,6], magnetometers [7,8], gravimeters [9,10] and gradiometers [11]. Many atom-based sensors achieve optimal performance by using laser-cooling techniques to create very cold atomic samples [12], and although efforts are ongoing to simplify and miniaturise such apparatus [13,14], laser cooling inevitably introduces significant experimental complexity and cost to the setup. In contrast, for atomic sensors where inhomogeneous Doppler broadening is not a problem, setups using

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Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

Cited by 12 publications

References 27 publications

Phase locking of THz QC-VECSELs to a microwave reference

Phase locking of THz QC-VECSELs to a microwave reference

A practical guide to terahertz imaging using thermal atomic vapour

A practical guide to Terahertz imaging using thermal atomic vapour

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